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Back again, but this time I believe I've done it! What I had missed was - it now seems so obvious of course - was a direct link between the pc and the device Kindle Fire HD. This was, I'd say, the key factor in resolving this issue, ie you have to access "Kindle for PC" site - either google or go use any apt link from Kindle Fire page. Tip: You'll know for sure when a blue/black kindle icon shows up in the save folder!)
BTW, this site uses 'whispersync', so no need for usb cable. :)
Now log in to your Amazon kindle fire a/c- thus go Your Account>Manage Kile - click Your Kindle Library. I wanted just one book from my eleven but I could have transferred the lot. So highlight the ebook you want and then slip across to the opposite right side of the name and you'll see an "action" prompt with options. in my case, it was the 1st of the tick boxes... I opted to deliver to my kindle name I had used when I registered my KF HD7. Check and confirm, click yellow box that says, "Deliver".
I know I haven't worded it too well, but it is one of those 'not as bad as it first looks' set-ups. Enjoy ... ;)